Lead zirconate titanate (PZT) is an oxide ceramic extensively used for its piezoelectric properties. Devices made from piezoelectric PZT include actuators and transducers as well as MEMS (microelectromechanical systems) devices such as lab on a chip (fluidic devices), surface acoustic wave (SAW) devices, tunable filters, RF switches, and all of the other devices MEMS promises. PZT is also widely used for ferroelectric applications such as non-volatile memories, electro-optic applications such as light valves, and applications in pyroelectric infrared detectors are also wide-spread.
Present processes for preparing PZT precursor materials suffer from a variety of different problems. One example of such a problem is the sale and use of water-reactive, limited shelf life PZT precursors that are solution processable only in toxic, flammable and hazardous solvents; see, for example, Alfa Aesar Product 36575 and 39758, Chemat Product PZT9103, and PZT Solution from Inostek, Korea.
Another example of such a problem is the processing of precursors in toxic solvents, employing large quantities of decomposable organics; see, for example, M. H. M. Zai et al, “Highly (111) oriented lead zirconate titanate thin films deposited using a non-polymeric route,” Thin Sold Films, Vol. 394, pp. 97–101 (2001), who discuss metal-organic decomposition (MOD) deposition methods for PZT and the use of their new precursor to make PZT thin films. It appears that their precursors are most likely water-reactive (although not water-soluble).
Still another example of process problems is the tedious multi-step route to a PZT MOD precursors disclosed by J. S. Wright and L. F. Francis in “Processing of Piezoelectric properties of MOD PZT film and PZT/Polymer composites,” Materials Research Society Symposium # 433, pp. 357–362 (1996).
The MOD process typically involves the synthesis of thin film ceramics from metal organic acid salts (mostly aliphatic acids such as neo-decanoic acid or 2-ethylhexanoic acid). The MOD process is described in, for example, (1) U.S. Pat. No. 5,721,009, “Controlled Carbon Content MOD Precursor Materials Using Organic Acid Anhydride,” issued to Thomas K. Dougherty et al on Feb. 24, 1998; (2) J. V. Mantese et al, “Metalorganic Deposition (MOD): A Nonvacuum, Spin-on, Liquid-Based, Thin Film Method,” MRS Bulletin, pp. 48–53 (October 1989); (3) WO 93/12538, “Process for Fabricating Layered Superlattice Materials,” filed in the names of Carlos A. Paz de Araujo et al, published on 24 Jun. 1993; (4) U.S. Pat. No. 5,434,102 (issued on Jul. 18, 1995) and U.S. Pat. No. 5,439,845 (issued on Aug. 8, 1995), to Hitoshi Watanabe al and both entitled “Process for Fabricating Layered Superlattice Materials and Making Electronic Devices Including Same”; and (5) G. M. Vest et. al, “Synthesis of Metallo-Organic Compounds for MOD Powders and Films,” Materials Research Society Symposium Proceedings, Vol. 60, pp. 35–42 (1986).
The present inventors and associates have continued their work in this area, culminating in (1) U.S. Pat. No. 6,054,600, “Non-Toxic Solvent Soluble Group IV and V Metal Acid Salt Complexes Using Polyether Acid Anhydrides,” issued to T. Kirk Dougherty et al on Apr. 25, 2000; (2) U.S. Pat. No. 6,303,804, “Environmentally Benign Bismuth-Containing Spin-on Precursor Materials,” issued to T. Kirk Dougherty et al on Oct. 16, 2001; and (3) U.S. Pat. No. 6,316,651, “Environmentally Benign Group II and Group IV or V Spin-on Precursor Materials,” issued to T. Kirk Dougherty et al on Nov. 13, 2001. The contents of these patents are incorporated herein by reference.
With specific regard to PZT materials, M. Klee et al in “Processing and Electrical Properties of PZT films. Comparison of metallo-organic decomposition and sol-gel processes,” Journal of Applied Physics, Vol. 72(4), pp. 1566–1576 (1992) compare the MOD and sol-gel process for PZT films.
Schwartz et.al in “A Comparison of Diol and Methanol-based Chemical Solution Routes for PZT Thin Film Fabrication,” Integrated Ferroelectrics, Vol. 18, pp 275–286 (1977) discuss propanediol-based precursors which have issues with uniformity, coating stability, and shelf life.
There remains a need for a soluble spin-on precursor which is compatible and soluble in non-toxic and environmentally benign solvents (including water), has unlimited stability and shelf life, and provides high quality PZT films and materials.